1. Field of the Invention
The present invention relates to a semiconductor device having a nonvolatile memory, a display device (a liquid crystal display, an organic EL display, etc.) using this semiconductor device, and a mobile device (a laptop computer, a cell-phone, a handheld terminal, etc.).
2. Description of the Related Art
A method for improving functions of a TFT device formed on an insulating substrate, by providing the device with a nonvolatile memory function, has attracted attention.
As the method for improving the functions of the TFT device described above, Japanese Unexamined Patent Publication No. 2002-110829 discloses the following method. This method will be described with reference to FIG. 22.
Japanese Unexamined Patent Publication No. 2002-110829 discloses that a TFT memory 90 includes a polysilicon layer 92 having regions of a source 92a, a channel 92b, and a drain 92c, respectively formed on a substrate 91, and gate oxide films (insulating films) 93, 95 formed on this polysilicon layer 92, and these gate oxide films 93, 95 contain a plurality of granular silicon particles 94 which capture charges of carriers.
However, it was found out to be difficult to be used as a display device by the method for capturing charges in a plurality of silicon particles.
Specifically, when the TFT memory is used as a display device, illumination such as a backlight or the like is needed. It was found out that, when this illuminating light is applied to the silicon particles, charges captured in the silicon particles are activated and thereby the charges escape out of the silicon particles. That is, it was found that in the case of the display device employing an illuminating light, charges cannot be held by capturing the charges in silicon particles.
So, experiments were performed using other charge holding films instead of the silicon particles and further using various memory holding films irrespective of the charge holding film, but these attempts have not lead to the solution of above-mentioned problem.